Rickets and metabolic bone disease — branching viva

Opening: classify and treat

The candidate opens by recognising nutritional vitamin D deficiency rickets and reads the biochemistry as calcipenic: a low or low-normal calcium, a low phosphate, a high alkaline phosphatase, and a raised parathyroid hormone. The 25-hydroxyvitamin D of 22 nmol per litre confirms deficiency against the 50 nmol per litre sufficiency threshold. The candidate explains that the classification separates calcipenic from phosphopenic rickets, and that the calcium and parathyroid hormone are read before the vitamin D level because they redirect the work-up. [1] [2]

The treatment of nutritional rickets is cholecalciferol 2000 IU daily for three months, or a stoss dose of 100 000 to 300 000 IU, together with oral calcium at 30 to 50 mg per kilogram per day elemental. The candidate explains why calcium is given with the vitamin D and sets the monitoring targets: alkaline phosphatase falling within four to six weeks, biochemistry normalising within months, and radiographic healing within three to six months. [1] [4]

Branch 1: the hypocalcaemic seizing infant

The examiner pivots to a six-month-old exclusively breastfed infant of a veiled mother presenting in winter with a seizure and a calcium of 1.4. The candidate recognises symptomatic hypocalcaemia as an emergency and gives intravenous calcium gluconate slowly with cardiac monitoring, using calcium rather than standard anticonvulsants to terminate the seizure. The candidate checks and corrects magnesium and avoids alkalosis. [3] [1]

The examiner probes the order of the drugs. The candidate explains that calcium is given before or with the cholecalciferol, never vitamin D alone, because high-dose vitamin D accelerates mineralisation and can deepen the hypocalcaemia through hungry bone. The emergency sequence is calcium, then oral calcium with cholecalciferol, then monitoring. [1] [4]

Branch 2: the child with a normal vitamin D and rickets

The examiner presents a two-year-old with bowing, short stature, dental abscesses, a normal calcium, a very low phosphate, a normal parathyroid hormone, and a normal 25-hydroxyvitamin D. The candidate reads this as phosphopenic rickets and identifies X-linked hypophosphataemia, an FGF23-mediated renal phosphate wasting from PHEX mutation. The candidate explains why vitamin D alone fails: the growth plate is starved of phosphate, the parathyroid axis is intact, and FGF23 suppresses the 1-alpha-hydroxylase. [2]

The treatment is oral phosphate in four to six daily doses with calcitriol, or burosumab from age one year. The candidate names the monitoring — phosphate, parathyroid hormone, renal ultrasound for nephrocalcinosis, growth, and the rickets radiograph — and explains that the limb deformity is addressed orthopaedically only once the metabolic disease is biochemically controlled. [2] [4]

Branch 3: the low alkaline phosphatase trap

The examiner asks what a low alkaline phosphatase means in a child who looks rachitic. The candidate answers hypophosphatasia, an inherited deficiency of tissue-nonspecific alkaline phosphatase, and explains that the alkaline phosphatase is inappropriately low rather than high. The candidate warns that bisphosphonates, which might be considered for the fractures, worsen the mineralisation defect and must be avoided, and that the treatment of the perinatal and infantile forms is enzyme replacement with asfotase alfa. [2]

Closing: prevention and the family

The examiner closes with the prevention programme. The candidate states the Global Consensus: 400 IU of vitamin D daily from soon after birth through adolescence for all infants and children, with particular attention to exclusively breastfed infants and to dark-skinned, veiled, and migrant families. The candidate explains that nutritional rickets in one child signals household risk, so the mother and siblings are screened and supplemented, and that the public-health and primary-care links sustain the supplement and address the determinants of adherence. [1] [3]